Problems after separation part of a mesh
It looks like a traditional normal issue, your faces are turned to the inside of the object.
It actually happens in both your objects, the 'shard' and the rest of the building.

Enter edit mode, select one of the darkened faces, press Shift + G to select grouped and choose Normal to select faces facing in the same direction (check the selection for unwanted non-darkened faces and deselect any that don't apply)

Then press Ctrl + N to make normals consistent across the surface.

Repeat for the red frames on the building too, they also seem to be facing inwards.